Interferometric lithography is one of the most flexible tools to generate microstructures that, by diffraction, produce polychromatic beams with spectral and angular properties of importance for optical security devices [11].
The way such microstructure diffracts light depends, on the one hand, on the shape, dimension and position of the light sources and, on the other hand, on the shape and dimensions of the spatial domain where interference takes place [12 and 13].
Many implementations of interferometric lithography have been described. The most important are:    point light sources producing spherical divergent waves, or plane waves obtained after collimation, the position and orientation of which can be controlled, the interference pattern being confined to the interior of a mask placed as close as possible to the plane of the photosensitive emulsion, typically of photographic nature;    two spherical waves interfering on their common focal volumes, said waves being formatted and shaped by two or more lenses properly located and adequately phased.
In the first case, a non-pixelated pattern is created. This type of implementation can be found in patents [3 to 6]. In the second case, the interference is restricted to dots of controlled shape, the pixels, usually arranged in random, polar or rectangular format. This type of implementation can be found in patents [7 to 10]. In the pixelated case, there is an inefficient utilisation of the photosensitive area, as the quality of the interference pattern—consequently the micro-relief that is generated after chemical or thermal processing—strongly depends on the Gaussian character of the laser beam. It is possible to overcome this problem by masking the Gaussian beam, using only the central part of it, but, in this case, the amount of available energy to expose the emulsion is reduced, leading to longer exposures. Visually, unless the density of dots is very high, the pixelated character is always visible, even if it does not jeopardise the quality and the security of the device. The larger the density of pixels, the longer will be the time needed to complete the exposure of all pixels that build up the desired shape. Manufacturing time is thus proportional to the total number of pixels.
In both cases, the interference pattern is very simple, basically consisting of quadratic fringes that can be approximated in most of the cases by linear and parallel fringes. Diffraction by linear phase gratings is well known, and the optical effects that can be obtained are limited [12].
In-plane holography (or focused holography) is a technique to encode images holographically, by creating an in-focus real image (object beam) on the photosensitive plane and introducing a reference beam—typically plane or spherical—thus generating an interference pattern within the area covered by the object beam [14]. On reconstruction, the spectral and angular properties of the diffractive beam are of interest. The problem with focused holography is that if the area of interest is to be covered by a several patterns, say N, each point receives energy from the reference beam N times, which severely degrades the modulation of the microstructure and reduces the diffraction efficiency, thus actually destroying it as a security device. If, somehow, the reference beam could be restricted to the exact area of the object beam, this problem would be overcome. One way to solve the problem is to place a mask with the desired shape in the image plane, as close as possible to the emulsion to avoid diffraction problems. The problem with this technique is that all the masks for the different patterns must be precisely aligned and separated from the emulsion with a thin layer of an optical liquid (to match the refractive index), making it very difficult (or even impossible) to use when a large number of masks is required. The total amount of time needed to complete the creation of the optical element is, in this case, proportional to the number of patterns (i.e. masks) needed to define the element image